Question

I am running an application through gdb and I want to set a breakpoint for any time a specific variable is accessed / changed. Is there a good method for doing this? I would also be interested in other ways to monitor a variable in C/C++ to see if/when it changes.

Answer 1

watch only breaks on write, rwatch let you break on read, and awatch let you break on read/write.

You can set read watchpoints on memory locations:

gdb$ rwatch *0xfeedface Hardware read watchpoint 2: *0xfeedface 

but one limitation applies to the rwatch and awatch commands; you can’t use gdb variables in expressions:

gdb$ rwatch $ebx+0xec1a04f Expression cannot be implemented with read/access watchpoint. 

So you have to expand them yourself:

gdb$ print $ebx  $13 = 0x135700 gdb$ rwatch *0x135700+0xec1a04f Hardware read watchpoint 3: *0x135700 + 0xec1a04f gdb$ c Hardware read watchpoint 3: *0x135700 + 0xec1a04f  Value = 0xec34daf 0x9527d6e7 in objc_msgSend () 

Edit: Oh, and by the way. You need either hardware or software support. Software is obviously much slower. To find out if your OS supports hardware watchpoints you can see the can-use-hw-watchpoints environment setting.

gdb$ show can-use-hw-watchpoints Debugger's willingness to use watchpoint hardware is 1.